In order to detect particles such as charged particles or biopolymers having kinetic energies, a particle detector is used for converting the arrival of ions into an electric signal. In mass spectrometry, ions are subjected to separation analysis in accordance with the ratio (m/z) of the mass (m) of the particles and the charge number (z) of the ions. In linear type time-of-flight mass spectrometers with the simplest structure, time-of-flight (TOF) of ions is represented as TOF=(m/2zeV)1/2l , and m/z can be determined by measuring the TOF. Here, e is the elementary electric charge; V is the acceleration voltage; and l is the flight length. Ion optics system such as a reflectron is used for reducing TOF variation caused by, for example, initial kinetic energy distribution in ionization and acceleration at a constant voltage.
Disadvantageously, the mass spectroscopy, in principle, cannot distinguish different ions that have the same m/z, because ions are separated according to m/z. As a simple example, N+ and N22+ (m/z=14) cannot be separated. In order to overcome this fundamental problem, particle detectors using superconductivity have been studied. A superconducting tunnel junction (STJ) can measure particle kinetic energies when the detector surface is impacted by a particle with a kinetic energy such as several keV to several ten keV that is adopted by many mass spectrometers. Since the kinetic energy of ions accelerated at a constant voltage is proportional to the charge states of the ions, z can be determined by kinetic energy measurement. As a result, particles such as N+ and N22+ having the same m/z but having different m can be separated (Non-Patent Document 1).
Nevertheless, the electric pulses that the STJ detectors generate at the time of particle impact have a rise time of about 100 ns and a decay time of several μs. Therefore the pulses have a wide pulse width that is about 3 orders of magnitude longer than the nano-second pulses of microchannel plate (MCP) or secondary electron multiplier (SEM),both of which operate at room temperature for detecting particles. For this reason, it is difficult to achieve a high mass resolution. In addition, an extremely low-temperature environment such as about 1 K or lower is required for the operation of the STJ detectors. Disadvantageously, a large cooling equipment would be required.
A superconducting stripline detector (SSLD) is composed of superconducting thin films processed into fine line patterns (Patent Documents 1 and 2, Non-Patent Documents 2, 3, and 4). In SSLD, electric pulses induced by particle impact reportedly have a rise time of 400 ps to several ns and a decay time of 500 ps to several ten ps, which are faster than STJ. The SSLD detectors, however, cannot perform separation analysis of different ions having the same m/z, because SSLD cannot measure individual particles kinetic energy like STJ.